Preheat the oven to 350 degrees. Spray 3-6″ circular pans with baking spray and set aside.
In a large bowl, combine the cake mix, buttermilk, vegetable oil, eggs, and extract until you don’t see anymore cake mix streaks. Divide it evenly among the 3 pans.
Bake the cake layers according to the package directions. Let them cool completely.
After the cake layers have cooled, beat the butter until smooth. Add in half of the powdered sugar and mix. Add in the rest of the sugar, heavy cream, and extracts. Mix until combined. Start with 3 tbsp of the heavy cream and add more until it’s spreadable with a spatula.
Stack the first layer on a plate or cake board. Spread the frosting on top (about 1/3-1/2 cup). It should only be a thin layer. Pipe a border of frosting around the perimeter of the cake layer. Basically you’re creating a “stopper” so that the filling doesn’t come out. Fill the center with fudge topping. It should be about as tall as the frosting border. Place the next cake layer on top and repeat the steps. Flip the last cake layer over and place it on top.
If you’d like the cake to be a bit easier to work with, refrigerate it for 30 minutes. After that spread the rest of the frosting on the top and sides of the cake. Use a cake scraper to smooth the sides and an offset spatula to smooth the top. It most definitely doesn’t have to be perfect since we are covering it with sprinkles!
When you’ve finished smoothing the cake, use your hand to add sprinkles around the sides and top. It can get messy, so be sure to place a rimmed baking sheet under the cake to catch any sprinkles that fall off! After the sprinkles are pressed in, place the cake in the fridge for about a half hour.
Once the cake has “set,” top it with your choice of decorations. When it gets to room temperature, slice and serve!